In general, a mask pattern formation technique employed in fabricating a semiconductor device has a significant affect on accuracy of a pattern formed on the semiconductor device. Particularly, if transmissivity of a mask pattern is not properly considered, a process margin is insufficient, and distortion of a pattern line width, such as shortening of linearity of the line width, occurs contrary to the original purpose of lithographic exposure, which results in deterioration of characteristics of the semiconductor device.
On the other hand, a semiconductor photolithography technique can properly adjust the amount of light passing through a mask by designing the mask elaborately. To this end, an optical proximity correction technique and a phase shifting mask technique have been introduced and various methods for minimizing light distortion due to deformation of a mask pattern have been developed.
Recently, with the development of chemically amplified resists that are very sensitive to light having a far-ultraviolet wavelength of 248 nanometers (nm) or 194 nm, practical techniques for enhancing a resolution have been introduced. Particularly, a technique for forming an auxiliary pattern, which is separated from a main pattern, such as a dummy pattern used to control an optical proximity effect, contributes to enhancement of resolution.
FIGS. 1a and 1b are diagrams showing a long line width and a short line width of an active transistor of a conventional semiconductor mask, respectively.
First, FIG. 1 shows a long line width of about 10 micrometers (μm) of the active transistor of the semiconductor mask. Typically, it is very important to secure the line width of the active transistor overlapping with a gate transistor for normal electrical operation of a gate device. A factor to determine such a characteristic electrically is a characteristic of a high voltage transistor.
The active transistor is composed of portions 2L and 2R in which contact holes 5L and 5R are to be formed and a transistor portion overlapping with a gate transistor. Here, a contour image 4a formed by an exposure apparatus is also shown. In addition, because the active transistor is sufficiently long in its longitudinal direction, a uniform line width is maintained long, as denoted by reference A.
FIG. 1b shows an active transistor related to a low voltage transistor, with a line width designed in the same way as FIG. 1 and with its length decreased. As shown in FIG. 1b, the length of the line width measured on a semiconductor substrate is greatly different from that of the line width in FIG. 1a. Namely, as the line width becomes shortened in its longitudinal direction, the optical proximity effect occurs greatly and the line width is formed somewhat large. Here, a contour image 4b is also shown in FIG. 1b. 
In this case, because a transistor portion 3 overlapping with the gate transistor is short in its longitudinal direction, a portion where the line width is uniformly maintained is very short, as denoted by reference B, and electrical characteristics also become unstable. Accordingly, it is difficult to properly fit a characteristic curve for a ratio of breakdown voltage (Vtl) to gate line width or a ratio of saturation current (Idsat) to gate line width.
FIG. 2 is a diagram illustrating optical proximity correction (OPC) applied to enhance a line width of a transistor in the prior art.
As shown in FIG. 2, although the OPC is applied to enhance the line width of the transistor, it is not easy to adjust the linearity of the line width uniformly. This is because patterns to which OPC for line width is applied in programs in common use are corrected by a single way independent of the length of the patterns. Accordingly, correction accuracy is deteriorated when the lengths of line width are different, as described above. Here, reference numeral ML denotes an OPC line width of the active transistor and reference numeral GW denotes a line width of the gate transistor. Particularly, when the line width GW of the gate transistor is wide and the line width ML of the active transistor is narrow, there is a problem in that electrical characteristic are deteriorated.